1. Field of the Invention
The present invention relates to a projecting lens, and, more particularly, to a projecting lens for a video projector for forming a bright and large image scope by projecting a CRT image.
2. Related Background Art
Recently, a video projector has been widely used for the purpose of forming a large-size TV image by reproduction. The performance of the projecting lens for the video projector is a critical factor when an excellent image quality is desired in the reproduced image. In order to obtain the bright image in a video projector of the type described above, a bright projecting lens of a large aperture is necessary. Furthermore, the angle of the projecting lens must be widened for the purpose of shortening the depth of the cabinet, that is, the projecting apparatus by shortening the distance from the surface of the CRT (Cathode Ray Tube), on which CRT image is positioned, to the screen.
In general, a video projector must have three projecting lenses so as to correspond to the CRTs of the three colors: B (blue), G (green) and R (red). In order to reduce the size and the weight of the lens for the purpose of reducing the cost and realizing the above-described high-level specification, a variety of projecting lenses using an aspherical lens have been disclosed.
A conventional projecting lens disclosed, for example, in U.S. Pat. No. 4,548,480 comprises, as shown in FIG. 1, three lenses. That is, it is constituted, when viewed from a screen (in a left portion of the drawing), a first lens L.sub.1 having positive refracting power, a biconvex second lens L.sub.2 having positive refracting power and a third lens L.sub.3 having negative refracting power and the strong curvature side of which faces the screen. Furthermore, an aspherical plastic lens is employed as the first lens L.sub.1 and the third lens L.sub.3 for the purpose of improving the image forming performance by correcting the aberrations. However, as shown in FIG. 2, a so-called warp of high-degree aberration is generated in the distortion in such a manner that a positive directional displacement occurs at the intermediate field angle and a negative directional displacement occurs at the outermost portion.
In the case where an excessive high-degree warp is, as shown in FIG. 2, generated in the distortion of a projecting lens of a three-tube type projector, the image designated by a dashed line and a continuous line of FIG. 4A becomes distorted as shown in FIG. 4B when images of cathode ray tubes T.sub.B, T.sub.G and T.sub.R are, as shown in FIG. 3, projected on a screen S.sub.C by using projecting lenses L.sub.B, L.sub.G and L.sub.R in accordance with a movement principle. As a result, the correction cannot be easily performed by the CRT side. Therefore, the color deviation is generated in the peripheral portion of the field angle, causing the quality of the projected image to be deteriorated.
In order to correct a high degree aberration in the distortion, the applicant of the present invention has disclosed a projecting lens in U.S. Pat. No. 4,770,513. According to that disclosure, the basic structure is arranged in such a manner that a meniscus lens the convex side of which faces the screen is disposed between the second lens group and the third lens group which serves as a field flattener. According to the projecting lens disclosed in U.S. Pat. No. 4,770,513, the high degree aberration in the distortion can be satisfactorily corrected with sufficiently securing the image brightness and the wide angle. However, it cannot be satisfactorily form a large magnification image. That is, when the magnification is enlarged, the various aberrations are also enlarged. As a result, the brightness and the wide angle cannot be easily realized in this case, causing a future necessity of sufficiently correcting the above-described various aberrations to be arisen.
There has been arisen a desire for a video projector to have a focusing mechanism capable of varying the projecting magnification by using a sole projecting lens in order to meet the status in which the video projector is used.
The conventional projecting lens for a video projector has been arranged in such a manner that focusing is performed by fixing a lens group including a field flattener and by integrally moving all of the lens groups that are positioned more adjacent to the screen than the fixed lens group in the direction of the optical axis.
In this case, although a fine adjustment of the projecting magnification can be satisfactorily performed at the time of performing the projection of an image to a predetermined screen, the excessive deterioration in the image quality due to the image surface movement on the screen cannot be prevented when the projecting magnification is desired to be changed in accordance with the change in the size of the screen. Furthermore, according to a focusing method of the type described above, the overall length of the lens is changed, causing a problem to be arisen in that the distance of the projection (the distance from the screen to the projecting lens) is undesirably changed at the time of the fine adjustment of the magnification.